Sandkings are also aliens from several episodes of The Outer Limits (and possibly originally from these stories-- I've never read them, so I don't know). Basically, the sandkings are extraterrestrialinsects similar to ants or termites.
A human scientist tries to study them by keeping a colony in a sand-filled tank in his garage. They seem only slightly intelligent at first, with a hive-mind something like ants. But the structures they build become more and more complex, until they build a larger-than-life model of his head.
At this point, he begins thinking of them as intelligent. He learns that if he destroys their buildings, they reconstruct them in amazingly short time. Their ranks even part almost respectively when he steps into the tank. He begins to think they may even worship him (the attention they pay him, and the monuments, would lead one to think that).
Their true intelligence is known at the end, however. Working together, the colony escapes. An entire swarm of aliens is unleashed on an environment with no defenses against their intelligence and efficiency. And who's going to believe a crazy scientists who claims to have found alien intelligence, and has no more proof than a few models in a sandbox?

Apparently this episode was based on George Martins's story. Thanks, aznin.